For many years powder metallurgy bodies fabricated from metals have found use in industry. Powder metallurgy compacts pressed from tantalum powder to approximately 1/4 to 3/4 of the theoretical density have been sintered to produce the tantalum capacitor anode bodies used to fabricate tantalum electrolytic capacitors. In order to facilitate the mass production of tantalum capacitor anodes, of which millions are fabricated daily, a binder or lubricant is usually mixed with the tantalum powder prior to the pressing step. This blending of tantalum powder and binder may be accomplished by one of two basic means: the binder may be employed in the form of a fine powder and mixed with the tantalum powder by dry-blending, physically shaking or tumbling the powders together, or the binder may be dissolved in a suitable solvent and the solution sprayed on, or tumbled with, the tantalum powder (called wet-blending), leaving binder-coated tantalum upon evaporation of the solvent.
After the tantalum powder/binder combination is pressed to form anode compacts, the binder has traditionally been removed via a vacuum distillation step at 200.degree. C. to 600.degree. C. prior to the high-temperature sintering step used to produce the finished capacitor anode bodies. The vacuum distillation binder removal step may also include the use of an inert "sweep" gas to assist in removing the binder from proximity to the anodes as it is volatilized.
The industry demand for ever smaller capacitors having increasing CV density and decreasing cost has led to the development of tantalum capacitor powders having an increasingly higher surface area per gram and smaller average particle size. In the 1960's, capacitor grade tantalum powder routinely bad a surface area of 0.05 square meter per gram and a CV product of 5,000 microfarad-volts (or microcoulombs) per gram. By the 1980's, capacitor grade tantalum powders were available with a surface area of 0.2 square meter per gram and a CV product of 20,000 microcoulombs per gram. Currently available capacitor grade tantalum powders may have as high as 0.5 to 1.0 square meter per gram surface area and 50,000 to 100,000 microcoulombs per gram. The average particle size of capacitor grade tantalum powder in the 1960's was in excess of 5 microns. The average particle size of the finest contemporary tantalum powders is below 0.2 micron and sub-micron tantalum powders have been prepared.
The increase in surface area and reduction in the average particle size of capacitor grade tantalum powders has made possible both the size and cost reductions in tantalum capacitors sought by industry. Unfortunately, the reduction in the pore size of tantalum powder metallurgy anodes resulting from the use of these finer tantalum capacitor powders makes the removal of the binder/lubricant progressively more difficult with decreasing particle size. Further complicating the binder removal is the increasing surface energy and resulting reactivity of the tantalum powder as the particle size is reduced. Thus, anodes pressed from high surface area tantalum powders have very small pores through which the binder vapor must diffuse and are composed of particles that become very reactive at traditional binder removal temperatures due to the high surface energy associated with the small radii of curvature of these fine particles. The result is that an increasingly high fraction of the binder reacts with the tantalum anode material during the binder removal step and may be detected by the standard carbon analysis tests used for reactive metals.
Experience indicates that carbon residues formed by reaction between carbonaceous binders and the tantalum powder comprising tantalum anode compacts give rise to flaws in the anodic oxide film dielectric formed via various anodizing processes. Flaws in the anodic oxide give rise to elevated leakage current levels both in liquid electrolyte solutions (used to test the anodes and to fill "wet-slug" capacitors) and in finished "solid" tantalum capacitors.
In order to help reduce the residual carbon content of powder metallurgy capacitor anode bodies pressed from high surface area tantalum powder (and the elevated leakage current and short-circuit problems associated with this residual carbon), capacitor manufacturers have employed various approaches to enhancing the binder removal process. Anodes may be processed in relatively small batches and spread out into relatively thin layers in order to minimize the diffusion path length which the binder vapor must transit to escape from the bulk of the anode bodies. As mentioned above, an inert "sweep" gas may be employed to help remove binder vapor from the vicinity of the anodes.
U.S. Pat. No. 4,664,883 describes a method of employing mixed binders in which one component exhibits relatively good binder properties, such as polyethylene oxide, and a second component which, while not having particularly good binder properties itself, decomposes at binder removal temperatures to yield a large quantity of gases which serve to help sweep the first binder component from the pores of the anode bodies. One disadvantage of this method is that the volatile binders described, ammonium carbonates, are hydroscopic and tend to absorb water during processing in humid environments, resulting in problems with powder flow.
Another relatively recently introduced binder material is polypropylene carbonate, which is sold by PAC Polymers under the name of "Q-PAC". This material thermally degrades in vacuum at approximately 250.degree. C. to yield propylene carbonate, propylene oxide, and carbon dioxide. This material has been found to leave much less carbonaceous residue within vacuum sintered powder metallurgy capacitor anodes pressed from high surface area tantalum powder than is found with traditional binders, such as stearic acid, CARBOWAX 8000, or ACRAWAX C. Unfortunately, polypropylene carbonate is very difficult to mill for dry-blending use due to the glass transition temperature of approximately 40.degree. C. (cryomilled powder tends to agglomerate into a solid mass unless stored and shipped under refrigeration). Due to the difficulties encountered in dry-blending polypropylene carbonate, the material is usually wet-blended with tantalum powder. The solubility of polypropylene carbonate is relatively high only in chlorinated solvents and acetone. The wet-blending of polypropylene carbonate on a manufacturing scale requires very thorough equipment design and careful plant operation to prevent ignition of or worker exposure to solvent fumes. Additionally, although polypropylene carbonate represents a definite improvement in ease of removal compared with traditional binder materials, such as stearic acid or ACRAWAX C, it remains very difficult to remove the last traces of polypropylene carbonate from powder metallurgy tantalum anode compacts.
A simple and straightforward approach to removing binders/lubricants from tantalum powder metallurgy anode compacts is described in U.S. Pat. No. 5,470,525. The inventors employ binders which are fairly water soluble and remove the binder following the anode compacting step via warm water washing. This method avoids reaction of the carbonaceous binder with the tantalum at traditional binder removal temperatures by avoiding temperatures sufficiently hot to decompose the binder. With this methodology, virtually all of the binder may be removed with little or no damage to the product. One disadvantage with this process is that manufacturing plants already equipped for vacuum/thermal binder removal must purchase and install equipment for the water washing process and the binder must be soluble in water and have the necessary lubriciousness and ability to agglomerate fine tantalum powders into small tantalum-binder agglomerates having superior flow and reduced dust formation compared to the tantalum powder, alone.
Additionally, in a plant in which both processes, vacuum/thermal binder removal and water wash binder removal, are operated care must be taken that pressed tantalum anodes containing binder suitable for one removal process are not inadvertently subjected to the other process. For example, tantalum anode bodies containing wet-blended polypropylene carbonate, which is removed thermally, will contain grossly excessive amounts of carbon following processing through a water wash process designed to remove polyethylene glycol 8000, and capacitor anode bodies compacted from high surface area tantalum powder containing polyethylene glycol 8000 will be badly contaminated with carbon if subjected to a vacuum/thermal binder removal process.
Ammonium carbonate or bicarbonate may be used as the sole binder and may be completely removed from powder metallurgy capacitor anode bodies compacted from high surface area tantalum powder by either removal method. These materials have several disadvantages, however. Due to their high vapor pressure, they must be dry-blended. The ammonium carbonates are mechanically weak substances and offer little or no lubrication value. Tantalum powder blended with the ammonium carbonates as the sole binder is subject to a great deal of dust generation and gives rise to excessive wear of the anode compacting presses due to the settling of fine, abrasive tantalum dust on the sliding and rotating parts of the equipment. The ammonium carbonates also evaporate from open containers of tantalum capacitor powder blended with them. The resulting variable binder content due to evaporation complicates control of the weight of the anodes produced.